This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present invention that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
It is now common practice for content provider to serialize audio-visual content which they distribute (screener, press previews, high definition video on demand, etc) in order to identify the source of a leak in a distribution network when copyrighted content is spotted on illegal distribution platforms (peer-to-peer, streaming, direct download). Digital watermarking is one technology that provides such serialization capabilities i.e. each recipient of the content owns, after watermark embedding, a slightly different version that uniquely identifies him/her.
Digital watermarking comprises error correction coding and modulation of the serialization message. The error correction code (ECC) is implemented to manage errors that may occur along the communication of the serialized content. The modulation is implemented through a spreading sequence to get a better resistance to communication distortion. ECC basically introduces redundancy to the message that is to be embedded. For instance, a m-symbols long serialization message is mapped to a n-symbols long encoded message (n>m). The spreading binary sequences are used to convey a single symbol of the encoded message. According to a particular embodiment, a lookup table maps each symbol to a s-bits long sequence. In the binary case, there are only two spreading sequences that are commonly either antipodal (all bits are different between the two sequences) or orthogonal (half the bits are orthogonal between the two sequences). Various embodiments of error correction coding and, modulation are described hereafter.
The watermark embedder is currently vowed to be deployed in end-user device firmware for security. Once released, it will not be possible to update the algorithm implemented on the device as if it was a simple piece of software. In other words, the error correction code, spreading sequence and serialization message need a priori to be “burnt” into the device. As a result, there is no easy means to update the algorithm once the devices are deployed. In other words, device is non-renewable regarding watermarking algorithm. Though, it could be desirable to modify such feature for improving the robustness of the watermarking system for instance after obtaining a better understanding of the piracy process.
However, when the serialization process is to be performed in low-cost consumer electronic devices, most of the computational cost is shifted in to a pre-processing step. The skilled in the art knows two-step watermark embedding process wherein watermarking assistance metadata are computed in a remote pre-processor. Watermarking assistance metadata comprise a value #Si associated with a symbol Si and to an embedding position. The watermark embedder present in the device is then fed with watermarking assistance metadata to perform elementary operations such as switch-based embedding. In case of operations for embedding symbols Si at position N, such metadata comprises data for setting the byte at position N in the input stream to value # Si if Si is to be embedded and iteratively for each symbol. However, known watermarking assistance metadata are limited and do not address the watermarking error correction coding and modulation previously described.
Thus known watermarking system raises the issue of modifying watermarking scheme, such as the modulation or error correction coding algorithm although not limited to those features, once devices are deployed.
Therefore, it is desirable for the embedder to be both (i) as simple as possible, and (ii) as flexible as possible. A watermark system is therefore needed that anticipate potential necessary changes in watermarking process and wherein the watermark embedder can accommodate as many of these changes without modifying the device firmware.